It's a violation of federal law.
Federal law prohibits ammunition sale or transfer to, and possession by, anyone who:
1. ...
2. ...
3. ...
4. ...
5. ...
6. is an illegal alien,
I guess we can assume it was also part of a criminal enterprise or conspiracy since they were "allegedly" delivering the ammo to a third party who may or may not have committed another crime by delivering it across the border. But it's a lot of assumptions as the article is short on facts.
I guess in the reporter's mind, an "assault rifle bullet" is one of those evil bullets that are much more dangerous than a regular bullet.